If it's mushroomed you sometimes have to file, but the outside, NOT the end. Now that you have it disassembled you can easily determine if the bushing is the problem. Merely reinstall the arm without the springs and without tightening the bolt. If either arm does not pivot very easily then the bushing has been distended. You can try to fix as I have indicated, but it's difficult to get just right. I believe the bushings were bronze (soft metal) so actually sandpaper is better than a file, and use only until the binding is reduced to an acceptable level. If you go too far you will have play in the arms.

Personally I would just try to find someone with an extra caliper.